Turning vanes required, yes or no?

I have a Carrier 5-ton gas furnace (58MTB-100) that needs a new supply plenum. The system currently splits into two 20" x 8" trunk ducts from the plenum, which is 20" x 20" at the top (I presume the height is irrelevant for this question).
If I simply replace the plenum (I am having a local metal-worker make one), do I need to add turning vanes in the new plenum? (The old plenum does not have any inside.)
If I do, how many, and how should they be mounted (the metal-worker is just that - he does not know HVAC specifics)?

Many thanks.

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If your airflow in the house was adequate before and your not making any changes I don't think it would make much difference.20x20 plenum to a 20x 8 split sounds a little tight to install proper turning vanes.

The airflow in the house has been a bit weak for a while, which is why I checked the furnace originally. The original plenum must have been made with scrap metal - it is rusty and corroded and full of holes, etc. My worry was that I would be dead-heading the flow into a hard 'T' without adding some sort of air channeling device. I assume that it would be easier to simply put a 'V' (or rounded 'V') in the top of the plenum than to worry about turning vanes, right?

While I have you here - are the 20" X 8" trunks the appropriate size to split that flow? (I am trying to learn a bit, in case I need to do some modification or have some modification done by a contractor.)

If you have a sheet metal mechanic making the fitting why not just have him make a 20 x 20 to 20 x 8 Wye with 90 deg short ells and put volume dampers in the 20 x 8 sides after the ells. He'll know what I mean.

Cold,
I will take your suggestion to the metal worker - I understand what you are suggesting, though I wonder why you suggest trunk dampers? (All of the takeoffs I have seen thus far have individual dampers in place - if the system is balanced between the two trunks, why add dampers to the trunks?)

Been,
Far too small from what I can tell - there are three 8" flex tubes feeding the return "box" at this point. Further, one of those three flex tubes enters the box and the air flow is almost immediately interrupted by a piece of sheet metal at an approximately 60 degree angle. I cannot imagine that, even flowing at an absolutely optimal rate, those three 8" flex ducts are feeding the furnace enough return air (the filter is a 16" x 20" x 1", if I recall correctly).

Actually, that raises a good question - should one repair/upgrade the return system or the supply system first?

Another question, how do you calculate the correct size of trunk duct based upon the size of the furnace? (The guy at a local HVAC supply store said that a 5-ton should push approximately 2000fpm, and a 20X8 trunk is good for about 1000fpm - how did you arrive at 24X8?)

It not based on FPM per say. FPM is foot per minute. And is a velocity measurement. Like miles per hours.

The system should have been sized to the house. And then the duct sized based on how it is ran. Which entails determining he longest TEL/total equivalent length.

Not knowing how your system is set up. A 20X 8 in theory will move 950CFM at a .1" pressure loss per 100' of TEL. The attachment of the 20X8 to the plenum will be equivalent to a min of 35" of length(closer to 50 foot). The take offs will be roughly 30 foot of equivalent length. The register boot will be equivalent to 15 foot.

Using 50 foot for the attachment to the plenum. 50+30+15=95 foot on the supply, without adding in the actual linear length yet. Or allowing for any reducing transitions. So both of your 20X8 trunks will be small for moving a total of 2000 CFM.

24X8 will have a PD in theory of .07" per 100 equivalent foot. So it allows for the transitions and the attachment to the plenum.

Your return needs more attention then the supply. 3-8" return ducts is not nearly enough. You might be getting 1200 CFM is they are real real short runs.

Been,
What effect does it have if I have more than a .1" pressure loss? How does that affect the heating of the home? (I will presume that the system was sized to the house when it was installed, as I do not actually know.)

Assuming PD = Pressure Drop, how does a 24X8 (with a PD of .07") make such a huge difference over a 20X8 (with an assumed PD of .1")? That is a teeny tiny difference.

Also, as a slight update, I had not looked closely enough at the returns - there are two 10" flex ducts and one 8" flex duct. That is still not enough, I am sure, but at least it is slightly better. The runs are not exactly short, either - the shortest (the 8") is nearly ten feet of flex duct (not fully stretched, either) that is then connected to a long run of pipe that leads upstairs. The two 10" ducts have runs of probably closer to twenty feet of flex (not fully stretched) that lead into return boxes set into the floor (one is 16X16, the other is much smaller at 10X8).

Your furnace probably has an ESP rating of .5" After subtracting all fixed PD devices(air filter, A/C coil, supply register, return grille, etc) you will probably have an ASP(available static pressure) of .2" or so.

FR is calculated using the ASP. And the TEL of your duct system(which is the supply and return combined). Which will probably come out close to 300 foot(290 to 375 are average duct system TELs).

So a .1" FR/PD per 100 foot would give you a pressure loss of .290(using 290' TEL), using .07" would give you a pressure loss/drop of .203" using the same length. Or a .087" difference.

A 10" flex is roughly 340CFM at a ." FR/pressure drop. Yours are obviously at a much higher loss, or your system would work at all.

Your returns need the most work. The 2-10" increased to 14" will help. And you will need to increase the size of the return grilles also. After doing that, you will get much more air from your supplies. They will probably be noisy though. And the closest ones to the furnace, will have the most air. And you may get some uneven heating and cooling in the house without fixing the supply trunks.

I have this reference available: http://xpedio.carrier.com/idc/groups...tb-1pdreva.pdf I think that will contain some of the information that will help me understand you and your suggestions. Also, for future reference, there is no air conditioner connected to the unit - that is a planned upgrade in the future.

Quote:

Originally Posted by beenthere

Your furnace probably has an ESP rating of .5" After subtracting all fixed PD devices(air filter, A/C coil, supply register, return grille, etc) you will probably have an ASP(available static pressure) of .2" or so.

In that PDF, on page 11, it states that, for heating, that furnace has a "Certified Ext (External?) Static Pressure of .2, and of .5 for cooling. (Out of curiosity, why are the two different, especially if the air conditioner requires a coil to be placed within the air flow?)

Is Static pressure measured as a positive (meaning, I can "push" the water column 1/5 inch higher) or as a negative (meaning, I can "pull" the water 1/5 inch lower)?

Quote:

Originally Posted by beenthere

FR is calculated using the ASP. And the TEL of your duct system(which is the supply and return combined). Which will probably come out close to 300 foot(290 to 375 are average duct system TELs).

FR?

Quote:

Originally Posted by beenthere

So a .1" FR/PD per 100 foot would give you a pressure loss of .290(using 290' TEL), using .07" would give you a pressure loss/drop of .203" using the same length. Or a .087" difference.

So .290/.203 = ~143% difference (or, .203 is ~70% of .290) - what sort of noticeable difference does that make to a person sitting in a room? Would they notice more air flow? More noise? More heat?

To be sure, I do believe you when you say X would be better than Y, I just want to understand why, both in terms of the physics and the physical.

Quote:

Originally Posted by beenthere

A 10" flex is roughly 340CFM at a ." FR/pressure drop. Yours are obviously at a much higher loss, or your system would work at all.

This sentence does not make sense. What PD? (May I assume .1"?) Also - mine are at a higher loss ... or my system would *not* work at all? <very confused>

Quote:

Originally Posted by beenthere

Your returns need the most work. The 2-10" increased to 14" will help. And you will need to increase the size of the return grilles also. After doing that, you will get much more air from your supplies. They will probably be noisy though. And the closest ones to the furnace, will have the most air. And you may get some uneven heating and cooling in the house without fixing the supply trunks.

I am hoping to completely re-do the return trunks - move them from their current positions (the two 10" are near the leaky front door area, the 8" runs from a buried [in a wall] duct that runs to the upstairs) to the central wall, where I would have a total of four 12X6 registers (two in each room that borders the central wall) and run that down into a common collector that then gets run to the return of the furnace. Once the downstairs [supply] ducting has been reconfigured, I was planning on adding a supply and return trunk up to the second floor and repeating the exercise.

I have this reference available: http://xpedio.carrier.com/idc/groups...tb-1pdreva.pdf I think that will contain some of the information that will help me understand you and your suggestions. Also, for future reference, there is no air conditioner connected to the unit - that is a planned upgrade in the future.

In that PDF, on page 11, it states that, for heating, that furnace has a "Certified Ext (External?) Static Pressure of .2, and of .5 for cooling. (Out of curiosity, why are the two different, especially if the air conditioner requires a coil to be placed within the air flow?)

The .2" is for furnaces being installed without an A/C coil. By using a low static of .2", they allow for the addition of an A/C coil.

Supply side static pressure on a furnace is measured between the A/C coil and the furnace.

Is Static pressure measured as a positive (meaning, I can "push" the water column 1/5 inch higher) or as a negative (meaning, I can "pull" the water 1/5 inch lower)?

Static pressure is measured as both. On a water manometer the amount of negative and positive moment, is combined. If a coulumn of watermoves up .2", the other side moved down .2", so it is combined, and totals .4".

FR?

FR, friction rate. The amount of static pressure lost per 100 foot of total equivalent length.

So .290/.203 = ~143% difference (or, .203 is ~70% of .290) - what sort of noticeable difference does that make to a person sitting in a room? Would they notice more air flow? More noise? More heat?

The blower can only move so much air against so much pressure. The higher the pressure its working against the less air it moves. The higher the static pressure, the more noise in the duct system. Lower air flow decreases heating and cooling efficiency. making it cost more to heat or cool your house.

To be sure, I do believe you when you say X would be better than Y, I just want to understand why, both in terms of the physics and the physical.

Take 2 garden hoses. 1 a 3/8" ID, the other a 5/8" ID. Connect the smaller one to your outdoor faucet, and open the faucet wide open. See how long it takes to fill a 5 gallon bucket. Then repeat with the larger hose. Same effect as duct work. The larger duct work moves more water easier. With in turn heats or cools a room/house quicker. And with less noise.

This sentence does not make sense. What PD? (May I assume .1"?) Also - mine are at a higher loss ... or my system would *not* work at all? <very confused>

If you had a manometer, and measured the static pressure at the return plenum where it attaches to, and then at the inside of the return grille. You would see a difference of probably .3 to .4" on those 10" flex returns. This would be the FR/friction rate of the duct moving what ever amount of air it is moving. In the case of a 10" flex, .3" FR would be 595CFM.

I am hoping to completely re-do the return trunks - move them from their current positions (the two 10" are near the leaky front door area, the 8" runs from a buried [in a wall] duct that runs to the upstairs) to the central wall, where I would have a total of four 12X6 registers (two in each room that borders the central wall) and run that down into a common collector that then gets run to the return of the furnace. Once the downstairs [supply] ducting has been reconfigured, I was planning on adding a supply and return trunk up to the second floor and repeating the exercise.

Thank you for all of your help!

make your return to the second floor much larger. if you want the A/C to work for up there when you add it.

Your welcome.

Take a piece of wood several inches wide and tall(plywood is fine). Put a foot on it, so that it can stand up by itself. Mark it from -3 to +3 in 1/10 or 1/8" increments(1/8 is easier, but not as accurate). Then attach a clear vinyl hose to it, in a U shape. that is long enough to have either of its ends inserted into a duct. Put water in it so both sides of the U have water at the 0 mark. You can now measure the static in your duct system. And see more of what i am talking about.